Adding mongodb URL support.

[davideagen]

Connections can be created with a full mongodb URL and valid mongodb urls can be parsed.

The slaveOk option has been implemented.

The rest of the features that can be specified in a mongodb:// URL (username,
password, database, multiple hosts, etc.) are not yet implemented and
warnings will be logged if attempts to use unimplemented funcationality are
made.

mongo.d changes:

connectMongoDB:
If the host field entered starts with 'mongodb://' try to construct the
MongoDB object with the URL parsing constructor. Otherwise use the
hostname/port constructor.

db.d changes:

Replaced m_host and m_port with a MongoConnectionConfig struct.
This struct holds all the configuration information that can be
specified via a mongodb:// URL.

Added a new constructor that parses a mongodb:// URL to create the
MongoConnectionConfig struct.

Modified the existing host/port constructor to instead use the host
and port to create a mongodb URL that is then passed to the new URL
constructor for processing.

Use the new MongoConnectionConfig constructor for MongoConnection
instead of the host/port constructor

connection.d changes:

Replaced m_host and m_port with a MongoConnectionConfig instance.

Added a constructor that takes a MongoConnectionConfig object.
This constructor warns (via logWarn) if features are specified in
the config that are not yet implemented.

Modified the host/port constructor to set a MongoHost instance
in the MongoConnectionConfig.

The query method has been modified to include the defQueryFlags
field of the MongoConnectionConfig struct. This implements the
new slaveOk option support.

A new function named parseMongoDBUrl (with unittest) parses a
mongodb:// URL and returns a MongoConnectionConfig object. If parsing
succeeds it returns true and false otherwise. Mongo url's are a
little different than regular URL's (multiple host/port entries, etc.)
so a dedicated parser seemed to make more sense than forcing mongo
specifics into the Url struct in url.d.

Added new structs MongoConnectionConfig and MongoHost.

[davideagen]

Existing functionality has been preserved so that code won't break with these changes.

However, one difference to note is that since the port is now passed as part of the URL string it can fail to be parsed. If that occurs (or if the URL can not be parsed for any reason) the MongoDB constructor will throw an Exception. It did not do that before.

[s-ludwig]

Looking good, thanks! I would just like to later rename MongoConnectionConfig to Mongo(Connection/Client/-)Settings and make it a class to be consistent with the other protocols (e.g. HttpServerSettings or SmtpClientSettings).

[davideagen]

Ok I'll make that change in my next pull request. I think it will be fairly easy to implement support for the database portion of the URL so I'm planning to do that too.